Anyway, here are direct links for different versions:
for 5.3.2:
http://koji.afroditi.hellasgrid.gr/packages/activemq/5.3.2/3.el5/x86_64/activemq-5.3.2-3.el5.x86_64.rpmhttp://koji.afroditi.hellasgrid.gr/packages/activemq/5.3.2/3.el5/x86_64/activemq-client-5.3.2-3.el5.x86_64.rpm
for 5.4.1:
http://koji.afroditi.hellasgrid.gr/packages/activemq/5.4.1/1.el5/x86_64/activemq-5.4.1-1.el5.x86_64.rpmhttp://koji.afroditi.hellasgrid.gr/packages/activemq/5.4.1/1.el5/x86_64/activemq-client-5.4.1-1.el5.x86_64.rpm
I'll work on a better solution with a public yum repo for this in the next days.